MRVP Test!?

Question

Why do you do fermentation tubes in addition to the MRVP media tests? Don't they test for the same thing?

Answer 1

The ability to ferment various carbohydrates to acidic products +/- gas can be tested with fermentation tubes. A pH indicator reveals acidic products (color change to YELLOW), and an inverted tube (Durham tube) traps gas. Results are reported as +/- acid and +/- gas.

Large groups of related organisms can be differentiated based on which of two different fermentation pathways they carry out. The MIXED ACID PATHWAY or the BUTYLENE GLYCOL pathway. Products of these paths are detected in the MR-VP test. If an organism uses the mixed acid pathway, the products are very low pH (~4), and are detected with the Methyl Red pH indicator (MR). If the butylene glycol pathway is used, a product of the path, acetoin, is detected in the VP test (Voges-Proskauer).

Essentially they are different tests to differentiate between bacteria based on their ability to ferment, how they ferment, and fermentation byproducts.